Active Matrix Type Display Apparatus and a Driving Device of a Load

ABSTRACT

An active matrix type display device includes a plurality of pixel circuits arranged in a matrix shape. The pixel circuit has a display device, a drive transistor of a first conductivity type for controlling a current flowing in the display device, a capacitor provided at a control electrode of the drive transistor, and a switch, connected to the control electrode of the drive transistor, for holding a drive control signal at the capacitor. The switch includes a switching transistor of the first conductivity type and a switching transistor of a second conductivity type, in which one main electrode of the switching transistor of the first conductivity type and one main electrode of the switching transistor of the second conductivity type are connected serially. One of the other main electrode of the switching transistor of the first conductivity type and the other main electrode of the switching transistor of the second conductivity type is connected to the control electrode of the drive transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an active matrix type display apparatus and adriving device of a load which are used for a television receiver,monitors of a computer, a cellular phone, a digital camera, a digitalvideo camera (camcorder), and the like, an exposure device for anelectrophotography printer, an exposure light source for aphotolithography, and the like. More particularly, the invention relatesto an active matrix type display apparatus (or device) and a drivingdevice (or apparatus) of an active device as a load which are preferablyused for a current driving type display device.

2. Related Background Art

As an active matrix electroluminescent display device, for example,there is a device disclosed in JP-A-2002-517806. FIG. 12 is a circuitdiagram of a conventional pixel circuit.

The circuit shown in FIG. 12 operates as follows. Switches (transistors)37 and 32 are closed. A switch (transistor) 33 is opened. An inputsignal Iin corresponding to a device current necessary for lightemission of an electroluminescent device 20 as an active device isinputted. A voltage across a capacitor 38 in a stationary state becomesa gate-source voltage necessary to drive a current flowing in a channelof a drive transistor 30. When the switches 37 and 32 are opened, thegate-source voltage which is determined in accordance with the inputsignal Iin is held in the capacitor 38.

Subsequently, when the switch 33 is closed, a drive current according toa level of the holding voltage flows in the electroluminescent device 20through the drive transistor 30, so that light is emitted. Referencenumeral 34 denotes a power line for setting a voltage (V2) on the anodeside of the electroluminescent device 20 and 31 indicates a power linefor setting a voltage (V1) on the source side of the transistor.

In JP-A-2002-517806, there is such a disclosure that n-type MOStransistors are used as transistors 32, 37, and 30 and a p-type MOStransistor is used as a transistor 33.

There is also known a pixel circuit in which a p-type MOS transistor isused as a drive transistor and a p-type MOS transistor is used as aswitching transistor for short-circuiting a circuit between a gate and adrain of the drive transistor. (Refer to the Official Gazette ofInternational Publication No. WO01/91094.)

In the active matrix type display device and the driving apparatus ofthe active device, there is still a room to be improved from twoviewpoints in which the drive current in the dark state is set to zeroand a fluctuation of the drive current due to an unnecessary leakagecurrent is prevented.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an active matrix typedisplay device and a driving apparatus of an active device, in which adrive current in a dark state is suppressed and an unnecessary leakagecurrent can be suppressed.

It is another object of the invention to provide an active matrix typedisplay apparatus and a driving device of a load, in which luminance ina dark state due to a fluctuation of a holding voltage accompanied bythe switching operation can be reduced and a fluctuation of theluminance due to an unnecessary leakage current can be suppressed.

According to the first invention of the present invention, there isprovided an active matrix type display apparatus having a plurality ofpixel circuits arranged in a matrix,

wherein the pixel circuit comprises:

a display element;

a drive transistor of a first conductivity type for controlling acurrent flowing in the display element;

a capacitor provided at a control electrode of the drive transistor; and

a switch, connected to the control electrode of the drive transistor,for holding a drive control signal at the capacitor, and

the switch includes a switching transistor of the first conductivitytype and a switching transistor of a second conductivity type in whichone main electrode of the switching transistor of the first conductivitytype and one main electrode of the switching transistor of the secondconductivity type are connected, and one of the other main electrode ofthe switching transistor of the first conductivity type and the othermain electrode of the switching transistor of the second conductivitytype is connected to the control electrode of the drive transistor.

According to the second invention of the present invention, there isprovided a driving device of a load, comprising:

a drive transistor of a first conductivity type for controlling acurrent flowing in the load;

a capacitor provided at a control electrode of the drive transistor; and

a switch, connected to the control electrode of the drive transistor,for holding a drive control signal at the capacitor, and

-   -   the switch includes a switching transistor of the first        conductivity type and a switching transistor of a second        conductivity type in which one main electrode of the switching        transistor of the first conductivity type and one main electrode        of the switching transistor of the second conductivity type are        connected, and one of the other main electrode of the switching        transistor of the first conductivity type and the other main        electrode of the switching transistor of the second conductivity        type is connected to the control electrode of the drive        transistor.

According to the third invention of the present invention, there isprovided an electro-luminescent display apparatus comprising:

a plurality of pixel circuits arranged in a matrix,

-   -   an electro-luminescent display element on said pixel circuit;

wherein said pixel circuit comprises:

a drive transistor of a first conductivity type for controlling acurrent flowing in said display element;

a capacitor provided at a control electrode of said drive transistor;and

a switch, connected to said control electrode of said drive transistor,for holding a drive control signal at said capacitor, and

-   -   said switch includes a switching transistor of the first        conductivity type and a switching transistor of a second        conductivity type in which one main electrode of said switching        transistor of the first conductivity type and one main electrode        of said switching transistor of the second conductivity type are        connected, and one of the other main electrode of said switching        transistor of the first conductivity type and the other main        electrode of said switching transistor of the second        conductivity type is connected to said control electrode of said        drive transistor.

According to the forth invention of the present invention, there isprovided a light emitting apparatus comprising:

a plurality of pixel circuits arranged in a matrix,

a light emitting element being connected with said pixel circuit;

wherein said pixel circuit comprises:

a drive transistor of a first conductivity type for controlling acurrent flowing in said display element;

a capacitor provided at a control electrode of said drive transistor;and

a switch, connected to said control electrode of said drive transistor,for holding a drive control signal at said capacitor, and

said switch includes a switching transistor of the first conductivitytype and a switching transistor of a second conductivity type in whichone main electrode of said switching transistor of the firstconductivity type and one main electrode of said switching transistor ofthe second conductivity type are connected, and one of the other mainelectrode of said switching transistor of the first conductivity typeand the other main electrode of said switching transistor of the secondconductivity type is connected to said control electrode of said drivetransistor.

As will be explained hereinafter with reference to FIG. 1, it ispreferable to construct in such a manner that

the other one of the other main electrode of a switching transistor M2 aof the first conductivity type and the other main electrode of aswitching transistor M2 b of the second conductivity type is connectedto one main electrode (drain) of a drive transistor M1, and

by turning on both of the switching transistor of the first conductivitytype and the switching transistor of the second conductivity type, thecontrol electrode (gate) and the one main electrode (drain) of the drivetransistor are short-circuited.

It is also preferable that the other main electrode of the switchingtransistor of the first conductivity type is connected to the controlelectrode of the drive transistor.

Further, it is also preferable that

a switching transistor M3 of the second conductivity type for selectinga row is provided between the one main electrode (drain) of the drivetransistor and a signal line (Idata, d(x, y)),

a switching transistor M4 of the first conductivity type for selectinglight emission is provided on a path of the current flowing in thedisplay device (EL), and

a control electrode of the switching transistor M2 b of the secondconductivity type, a control electrode of the switching transistor M3for selecting the row, and a control electrode of the switchingtransistor M4 for selecting the light emission are connected in commonto a second scan signal line.

As shown in FIG. 2, it is also preferable that after time when theswitching transistor of the first conductivity type is changed from ONto OFF (timing when P2 is changed from the low level to the high level),the switching transistor of the second conductivity type is changed fromON to OFF (P2 is changed from the high level to the low level).

Or, as will be explained hereinafter with reference to FIG. 9, it isalso preferable that the other main electrode of the switchingtransistor of the second conductivity type is connected to one mainelectrode of the drive transistor.

Also in the case of FIG. 9, it is also preferable that the switchingtransistor of the second conductivity type for selecting the row isprovided between the one main electrode of the drive transistor and thesignal line,

the switching transistor of the first conductivity type for selectingthe light emission is provided on the path of the current flowing in thedisplay device, and

the control electrode of the switching transistor of the secondconductivity type, the control electrode of the switching transistor forselecting the row, and the control electrode of the switching transistorfor selecting the light emission are connected in common to the secondscan signal line.

As will be explained hereinafter with reference to FIGS. 10 and 11, itis also preferable that the other one of the other main electrode of theswitching transistor of the first conductivity type and the other mainelectrode of the switching transistor of the second conductivity type isconnected to an output terminal of a voltage buffer X, and

-   -   an input terminal of the voltage buffer is connected to the        signal line (Idata, d(x, y)).

As shown in FIG. 10, it is also preferable that the other one of theother main electrode of the switching transistor of the firstconductivity type and the other main electrode of the switchingtransistor of the second conductivity type is connected to an outputterminal of a source follower circuit and an input terminal of thesource follower circuit is connected to the signal line.

As shown in FIG. 11, it is also preferable that the other one of theother main electrode of the switching transistor of the firstconductivity type and the other main electrode of the switchingtransistor of the second conductivity type is connected to an outputterminal of a feedback type operational amplifier and an input terminalof the feedback type operational amplifier is connected to the signalline.

According to the present invention, it is also preferable that the drivetransistor of the first conductivity type and the switching transistorof the first conductivity type are p-channel type thin film transistorsand the switching transistor of the second conductivity type is ann-channel type thin film transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of a construction of a pixelcircuit according to the first embodiment of the invention;

FIG. 2 is a timing chart for explaining the operation of the pixelcircuit according to the first embodiment of the invention;

FIG. 3 is a diagram showing a construction of an active matrixelectroluminescent display device according to the invention;

FIG. 4 is a timing chart for explaining the generating operation of linesequential data line signals;

FIG. 5 is a timing chart for explaining the generating operation of rowscan signals of the pixel circuit shown in FIG. 1;

FIG. 6 is a diagram showing a construction of a pixel circuit of acomparison example regarding the first embodiment of the invention;

FIG. 7 is a timing chart for explaining the operation of the pixelcircuit in FIG. 6;

FIG. 8 is a timing chart for explaining the generating operation of rowscan signals of the pixel circuit shown in FIG. 6;

FIG. 9 is a diagram showing an example of a construction of a pixelcircuit according to the second embodiment of the invention;

FIG. 10 is a diagram showing an example of a construction of a pixelcircuit and a voltage buffer circuit according to the third embodimentof the invention;

FIG. 11 is a diagram showing a modification of the pixel circuit and thevoltage buffer circuit according to the third embodiment of theinvention;

FIG. 12 is a circuit diagram of a conventional pixel circuit;

FIGS. 13A, 13B, 13C and 13D are diagrams showing manufacturing steps ofthe portions of a pMOS transistor M2 a and an nMOS transistor M2 b whichare used in the invention; and

FIG. 14 is a cross sectional view showing a construction of an ELdisplay device manufactured by the manufacturing method of FIG. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a pixel circuit shown in FIG. 6, the inventors et al. of the presentinvention manufactured the pixel circuit by using: a p-type MOStransistor using low-temperature polysilicon as a drive transistor; andan n-type MOS transistor using low-temperature polysilicon as aswitching transistor for short-circuiting a circuit between a gate and adrain of the drive transistor. In this case, in a black display mode,sufficient darkness is not accomplished due to a drop of a voltage heldbetween the gate and a source accompanied by the switching operation.This results in reduction of contrast in the case of using the pixelcircuit for a display device, an exposure device, or an exposure lightsource.

A pixel circuit is also manufactured by using: a p-type MOS transistorusing low-temperature polysilicon as a drive transistor; and a p-typeMOS transistor using low-temperature polysilicon as a switchingtransistor for short-circuiting a circuit between the gate and the drainof the drive transistor. In this case, although the sufficient darknessis obtained in the black display mode, it has been found that a leakagecurrent through the switching transistor occurs. Preferred embodimentsto solve such problems will be described in detail hereinbelow withreference to the drawings.

According to the preferred embodiments of the invention, the reductionof the contrast due to the fluctuation in holding voltage accompanied bythe switching operation can be suppressed and the generation of theleakage current can be suppressed.

FIRST EMBODIMENT

FIG. 1 is a diagram showing an example of a construction of a pixelcircuit according to the first embodiment of the invention. FIG. 2 is atiming chart for explaining the operation of the pixel circuit of FIG.1.

FIG. 3 is a constructional diagram showing a construction of an activematrix electroluminescent display device according to the invention.

In FIG. 3, reference numeral 1 denotes pixel circuits arranged in amatrix shape; 2 voltage-current converting circuits as signal line drivecircuits which are connected to the pixel circuits 1 arranged in thecolumn direction and supply line sequential data line current signalsIdata to the pixel circuits 1 through signal lines d(x, y); 3 columnshift registers connected to the voltage-current converting circuits 2;and 4 row shift registers serving as scan line drive circuits which areconnected to the pixel circuits 1 arranged in the row direction andoutput a row scan signal P1 and a row scan signal P2 to the pixelcircuits 1. A plurality of pixel circuits 1 are arranged in a matrixshape and construct a pixel unit.

FIG. 4 is a timing chart for explaining the generating operation of theline sequential data line signals. A clock signal K is inputted to thecolumn shift registers 3. A video signal (Video) is inputted to thevoltage-current converting circuits 2. The voltage-current convertingcircuits 2 supply the line sequential data line current signals Idata(d(n−1) to d(n+1)) to the columns of the pixel circuits on the basis ofsignals SP(n−1) to SP(n+1) from the column shift registers 3.

FIG. 5 is a timing chart for explaining the generating operation of rowscan signals of the pixel circuit shown in FIG. 1, which will beexplained hereinafter. A clock signal LK is inputted to the row shiftregisters 4. The row scan signals P1 (P1(m−1) to P1(m+1)) and the rowscan signals P2 (P2(m−1) to P2(m+1)) are sequentially outputted from therow shift registers 4 to the rows of the pixel circuits 1.

FIG. 6 is a diagram showing a construction of a pixel circuit of acomparison example regarding the embodiment of the invention. FIG. 7 isa timing chart for explaining the operation of the pixel circuit in FIG.6. FIG. 8 is a timing chart for explaining the generating operation ofrow scan signals of the pixel circuit shown in FIG. 6.

A fundamental construction regarding the programming operation of thecurrent signals Idata in the comparison example of FIG. 6 issubstantially the same as that of the pixel circuit shown in FIG. 12. InFIG. 12, the switch 32 can be regarded as an nMOS transistor M2, theswitch 37 can be regarded as an nMOS transistor M3, and the switch 33can be regarded as a pMOS transistor M1.

First, prior to explaining the embodiment, the comparison example willbe described for enabling the construction of the invention to be easilyunderstood.

The operation of the pixel circuit shown in FIG. 6 of x-column and y-rowin the case of displaying the pixel in a position of x-column and y-rowin the black display mode will now be considered. In FIG. 7, when therow scan signal P1 is set to the high level, the nMOS transistor M3serving as a switch for a first program (for selecting the row) isturned on and the pMOS transistor M4 serving as a switch for selectingthe light emission is turned off. When the row scan signal P2 is set tothe high level, the nMOS transistor M2 serving as a switch for a secondprogram is turned on. A voltage of a capacitor C1 connected to a gate ofthe pMOS transistor M1 serving as a drive transistor is set to agate-source voltage enough to allow a current for driving anelectroluminescent device EL as an active device to flow through thepMOS transistor M1. Subsequently, when the row scan signal P2 is set tothe low level, the nMOS transistor M2 serving as a switch for the secondprogram is turned off and the voltage of the capacitor C1 is held. Aperiod of time so far is called a programming period.

After that, when the row scan signal P1 is set to the low level, thenMOS transistor M3 serving as a switch for the first program (forselecting the row) is turned off and the pMOS transistor M4 serving as aswitch for selecting the light emission is turned on. The supply of thedrive current to the electroluminescent device EL is controlled by agate potential of the drive transistor M1 and the current flowing in theelectroluminescent device EL is controlled. A period of time duringwhich the electroluminescent device EL is emitting the light (non-lightemission in the case of the black display data) is called a lightemitting period.

In the pixel circuit of FIG. 6, to stably hold the voltage of thecapacitor C1, an nMOS transistor whose leakage current is small is usedas a transistor serving as a switch for the second program. This isbecause when the leakage current is large, the drive current in thelight emitting period fluctuates.

However, as shown in FIG. 7, when a gate of the nMOS transistor M2 isswitched from the high level to the low level in the programming period,the electric potential of the capacitor C1 is oscillated due to aparasitic capacitance between the gate and drain of the nMOS transistorM2, a voltage Vd(x, y) to be held drops by an amount of V_(M), so thatthe current flowing in the drive transistor M1 is increased by an amountof I_(M). In such a case, even in the case where the pixel of x-columnand y-row is in the black display mode in the light emitting period, asmall current flows in the pMOS transistor M4 due to the drop of thegate potential (holding voltage). Consequently, a small light emissionis observed in spite of the black display mode. In other words, thedarkest state cannot be normally obtained and it is difficult to assurethe contrast.

In the embodiment, as shown in FIG. 1, the switch for the second programconnected between the capacitor C1 (gate of the drive transistor M1) andthe drain of the drive transistor M1 is constructed by a pMOS transistorM2 a and an nMOS transistor M2 b which are serially connected. That is,the construction of the pixel circuit of FIG. 1 differs from that of thecomparison example of FIG. 6 with respect to a point that the nMOStransistor M2 in FIG. 6 is replaced by two switching transistors of thedifferent conductivity types which are serially connected (pMOStransistor M2 a and nMOS transistor M2 b).

In the case of holding the voltage into the capacitor C1, since a gateof the pMOS transistor M2 a is changed from the low level to the highlevel in the programming period, on the contrary to the potential changeshown in FIG. 7, the electric potential of the capacitor C1 isoscillated due to the parasitic capacitance between the gate and drainand the voltage Vd(x, y) to be held is increased by an amount of V_(L),so that the current flowing in the drive transistor M1 is decreased byan amount of I_(L). Thus, the pixel current flowing in the black displaymode can be eliminated or reduced.

As for the line sequential data line current signals, although the zerocurrent is preferable in the black display mode, it is actuallydifficult to realize the zero current in terms of the circuitconstruction. If the current of the line sequential data line currentsignals is not equal to zero, a pixel current Id cannot be set to zero.In the construction of FIG. 6, when the nMOS transistor M2 is turnedoff, the voltage held in the capacitor C1 is oscillated and drops.Therefore, the pixel current Id rises further and it becomes furtherdifficult to set the pixel current Id to zero.

If one of the switches for the second program is constructed by the pMOStransistor like an embodiment, since the direction of the electricpotential in the capacitor C1 to be oscillated is reversed, even if thecurrent of the line sequential data line current signals is not equal tozero, the pixel current Id in the black display mode can be set to zeroor sufficiently reduced due to the increase in electric potential of thecapacitor C1.

Although the leakage current of the pMOS transistor used in theembodiment is larger than that of the nMOS transistor, by adding thenMOS transistor M2 b in series with the pMOS transistor like anembodiment, the leakage current is suppressed and the holding voltage Vdin the light emitting period can be stabilized.

Although the capacitor C1 can be individually formed as a capacitancedevice, it is not always necessary to be formed as a device but aparasitic capacitor formed between the gate and the drain (overlappedcapacitor of a gate electrode and a drain electrode, or the like) can bealso used.

FIGS. 13A to 13D are diagrams showing manufacturing steps of theportions of the pMOS transistor M2 a and the nMOS transistor M2 bserving as field effect type thin film transistors using low temperaturepolysilicon. FIG. 14 is a cross sectional view showing a construction ofan EL display device manufactured by the manufacturing method of FIGS.13A to 13D.

As shown in FIG. 13A, after an amorphous silicon layer is deposited ontoa glass substrate 100 by using a plasma CVD method, it is thermallytreated (laser annealing) by a laser beam or the like to thereby form apolysilicon layer. By patterning it, polysilicon layers for the pMOStransistor M2 a and the nMOS transistor M2 b is formed.

In this step, it is also possible to execute channel doping of dopant(phosphorus or boron) for exhibiting the conductivity type opposite tothat of the source-drain to at least one of the polysilicon layers asnecessary and adjust a threshold value.

Subsequently, as shown in FIG. 13B, a gate insulating film 102 of SiO₂,SiN, or the like is formed, polysilicon is formed and patterned, and agate electrode 103 is formed.

As shown in FIG. 13C, p-type impurities (phosphorus or the like) andn-type impurities (boron or the like) are ion-implanted and thermallydiffused and source-drain regions 105 of the pMOS transistor M2 a andsource-drain regions 104 of the nMOS transistor M2 b are formed.

As shown in FIG. 13D, after the insulating film of SiO₂, SiN, or thelike is formed, contact-holes are formed and a metal layer serving assource and drain electrodes and wirings is laminated and patterned.After that, a planarized film 106 is formed, through-holes are formed,an anode electrode (not shown) is formed and patterned, subsequently, anelectroluminescent layer (EL layer) 107 is formed by evaporationdeposition, ink-jet (liquid-jet) deposition or the like, and an ITO film108 is formed. Whereby, electroluminescent light emitting elementsdisposed on pixel circuits and being electrically connected thereto.Preferably, the EL layer comprises a plurality of layers constructingwhat is called an organic LED. Further, it is preferable that the ELlayer is separated and independent every pixel.

As shown in FIG. 14, the substrate 100 is sealed to a glass vessel 109by glass, thereby completing the EL display device.

According to the embodiment described above, the transistor in which theleakage current of the pMOS transistor is extremely larger than that ofthe nMOS transistor is manufactured. However, there is also a case wherethe transistor in which the leakage current of the nMOS transistor islarger than that of the pMOS transistor is manufactured in dependence onthe manufacturing processes. The invention is also suitably applied tosuch a case.

SECOND EMBODIMENT

FIG. 9 is a diagram showing an example of a construction of a pixelcircuit according to the second embodiment of the invention. The signalsfor making the pixel circuit operative are substantially the same asthose shown in FIG. 2. In the first embodiment, as shown in FIG. 1, thepMOS transistor M2 a is connected to the gate of the pMOS transistor M1and the nMOS transistor M2 b is connected to the drain of the pMOStransistor M1. However, in the second embodiment, as shown in FIG. 9,the nMOS transistor M2 b is connected to the gate of the pMOS transistorM1 and the pMOS transistor M2 a is connected to the drain of the pMOStransistor M1.

Other constructions are similar to those in the first embodiment.

According to the pixel circuit with such a connecting form as well, theoperation and effects similar to those in the first embodiment can beobtained by a feedthrough of the nMOS transistor M2 b in the ON state.

THIRD EMBODIMENT

To make the pixel circuit shown in FIG. 1 or 9 operative, it is requiredthat the capacitor C1 and the parasitic capacitor due to theintersection or the like of the wirings are charged by the linesequential data line current signals. Although it is required to controlthe pixel circuit 1 by a small current in order to obtain a highcontrast ratio, there is a case where the charging time of the capacitorC1 and the parasitic capacitor becomes long due to the small current andthe small current setting operation for one horizontal scan period isinsufficient. Such a drawback becomes a further remarkable problem in aTFT circuit in which a threshold voltage variation ΔVth of the currentdrive transistor M1 of the pixel circuit 1 of each row is large. Sincethe capacitor C1 has to hold the current driving operation for one frameperiod of a video signal Video, its capacitance value cannot be set to asmall value.

The embodiment intends to provide a construction in which even if thecurrent signal that is inputted to the pixel circuit is a small current,the setting operation time can be shortened. The construction in whichthe voltage buffer is added to the pixel circuit as in the embodimenthas been disclosed in, for example, JP-A-2004-118181. A source-followercircuit or a feedback type operational amplifier can be used as avoltage buffer.

FIG. 10 is a diagram showing an example of a construction of a pixelcircuit and a voltage buffer-circuit according to the third embodimentof the invention.

In the embodiment, a voltage buffer X whose output voltage is determinedby an input voltage is provided every pixel circuit column. The voltagebuffer X is constructed by a source-follower circuit. Thesource-follower circuit comprises a pMOS transistor and a currentsource. An output terminal side of the voltage buffer X (connectingpoint of the pMOS transistor and the current source) is connected to thenMOS transistor M2 b and an input terminal side (gate of the pMOStransistor) is connected to the input signal line of the line sequentialdata line current signals Idata.

Other constructions are similar to those of the first embodiment.

As shown in FIG. 11, a feedback type operational amplifier can be alsoused as a voltage buffer in place of the source-follower circuit.

Also in this case, constructions other than the portions shown in FIG.11 are similar to those of the first embodiment.

According to the embodiment, since the voltage of the same electricpotential as that of the drain of the drive transistor M1 is held in thecapacitor C1 by the operation of the voltage buffer, the currentcorresponding to the current signal Idata which is programmed in thepixel circuit can be allowed to flow in the active device EL.

In this manner, the driving in which the adverse influence by avariation in characteristics of every pixel of the drive transistor issuppressed can be executed in a manner similar to those of the first andsecond embodiments.

Although the example in which the pMOS transistor is used as a drivetransistor M1 has been mentioned in each of the foregoing embodiments,in the case of using the nMOS transistor as a drive transistor M1, it issufficient to reverse the polarities of the active device, signals, andpower source. Specifically speaking, it is preferable that a drain ofthe nMOS transistor for driving is connected to a cathode side of an LEDas an active device, an anode of the LED is connected to a highpotential power source, and a source of the nMOS transistor for drivingis connected to a low potential power source.

Various emitting devices such as inorganic LED, organic LED (organicEL), electron emitting device, semiconductor laser, and the like can beused as active devices (loads) which are used in the invention.

Further, although the invention is suitably used for the crystallinethin film transistor represented by what is called a low temperaturepolycrystalline silicon, the circuit construction of the invention canbe constructed by an amorphous silicon TFT, a mono-crystalline siliconTFT, a high temperature poly-crystalline silicon TFT, or the like.

Particularly, the invention is suitably applied to use for an activematrix type display device of a current driving type light emittingdevice such as an electroluminescent device (EL device) or the like.

This application claims priority from Japanese Patent Application No.2004-186483 filed on Jun. 24, 2004, which is hereby incorporated byreference herein.

1. An active matrix type display apparatus having a plurality of pixelcircuits arranged in a matrix, wherein said pixel circuit comprises: adisplay element; a drive transistor of a first conductivity type forcontrolling a current flowing in said display element; a capacitorprovided at a control electrode of said drive transistor; and a switch,connected to said control electrode of said drive transistor, forholding a drive control signal at said capacitor, and said switchincludes a switching transistor of the first conductivity type and aswitching transistor of a second conductivity type in which one mainelectrode of said switching transistor of the first conductivity typeand one main electrode of said switching transistor of the secondconductivity type are connected, and one of the other main electrode ofsaid switching transistor of the first conductivity type and the othermain electrode of said switching transistor of the second conductivitytype is connected to said control electrode of said drive transistor. 2.An apparatus according to claim 1, wherein the other one of the othermain electrode of said switching transistor of the first conductivitytype and the other main electrode of said switching transistor of thesecond conductivity type is connected to one main electrode of saiddrive transistor, and by turning on both of said switching transistor ofthe first conductivity type and said switching transistor of the secondconductivity type, said control electrode and said one main electrode ofsaid drive transistor are short-circuited.
 3. An apparatus according toclaim 2, wherein the other main electrode of said switching transistorof the first conductivity type is connected to said control electrode ofsaid drive transistor.
 4. An apparatus according to claim 3, wherein aswitching transistor of the second conductivity type for selecting a rowis provided between said one main electrode of said drive transistor anda signal line, a switching transistor of the first conductivity type forselecting light emission is provided on a path of the current flowing insaid display element, and a control electrode of said switchingtransistor of the second conductivity type, a control electrode of saidswitching transistor for selecting the row, and a control electrode ofsaid switching transistor for selecting the light emission are connectedin common to a second scan signal line.
 5. An apparatus according toclaim 2, wherein after the time when said switching transistor of thefirst conductivity type is changed from ON to OFF, said switchingtransistor of the second conductivity type is changed from ON to OFF. 6.An apparatus according to claim 2, wherein the other main electrode ofsaid switching transistor of the second conductivity type is connectedto said control electrode of said drive transistor.
 7. An apparatusaccording to claim 6, wherein a switching transistor of the secondconductivity type for selecting a row is provided between said one mainelectrode of said drive transistor and a signal line, a switchingtransistor of the first conductivity type for selecting light emissionis provided on a path of the current flowing in said display device, anda control electrode of said switching transistor of the secondconductivity type, a control electrode of said switching transistor forselecting the row, and a control electrode of said switching transistorfor selecting the light emission are connected in common to a secondscan signal line.
 8. An apparatus according to claim 1, wherein theother one of the other main electrode of said switching transistor ofthe first conductivity type and the other main electrode of saidswitching transistor of the second conductivity type is connected to anoutput terminal of a voltage buffer, and an input terminal of saidvoltage buffer is connected to a signal line.
 9. An apparatus accordingto claim 1, wherein the other one of the other main electrode of saidswitching transistor of the first conductivity type and the other mainelectrode of said switching transistor of the second conductivity typeis connected to an output terminal of a source-follower circuit, and aninput terminal of said source-follower circuit is connected to a signalline.
 10. An apparatus according ot claim 1, wherein the other one ofthe other main electrode of said switching transistor of the firstconductivity type and the other main electrode of said switchingtransistor of the second conductivity type is connected to an outputterminal of a feedback type operational amplifier, and an input terminalof said feedback type operational amplifier is connected to a signalline.
 11. An apparatus according to claim 1, wherein said drivetransistor of the first conductivity type and said switching transistorof the first conductivity type are p-channel type thin film transistorsand said switching transistor of the second conductivity type is ann-channel type thin film transistor.
 12. A driving device of a load,comprising: a drive transistor of a first conductivity type forcontrolling a current flowing in said load; a capacitor provided at acontrol electrode of said drive transistor; and a switch, connected tosaid control electrode of said drive transistor, for holding a drivecontrol signal at said capacitor, and said switch includes a switchingtransistor of the first conductivity type and a switching transistor ofa second conductivity type in which one main electrode of said switchingtransistor of the first conductivity type and one main electrode of saidswitching transistor of the second conductivity type are connected, andone of the other main electrode of said switching transistor of thefirst conductivity type and the other main electrode of said switchingtransistor of the second conductivity type is connected to said controlelectrode of said drive transistor.
 13. A device according to claim 12,wherein a plurality of circuits each having said drive transistor of thefirst conductivity type, said capacitor, and said switch are arranged ina matrix. 14-15. (canceled)